Originally Posted by Otter
Aside: Perfect paintballs at 280 FPS shoots the same irregardless of pressure, pressure does not affect distance...
You're kind of over constrained there. Yes 280 is 280. However, all things being equal in a mag, higher dump chamber pressure means more air going out the barrel (and more velocity). Since the air doesn't help close the valve like a sheridan or nelson, higher pressure is always more volume out.
With air and a fixed dump chamber, you can use ideal gas laws.
That's the calculation at the beginning and end. In the middle is the gas leaving the chamber and going through various restrictions and leaks. TK measured the pressure immediately behind the ball on a level 7 at about 60 psi. There are other dynamics involved, but that gives you an idea of what the burst of air is doing.
The equation above says that you need a certain amount of pressure and volume together to equal what you spend on a single shot. The magic box trades more volume for less pressure. The problem with lower pressure is that it can be too low. Most acceleration occurs near the bolt face, so as you try to launch with lower and lower pressures, you can't push enough air through the power tube to keep up with the expanding volume behind the ball and your losses around the ball and out of the feed neck.
With a longer barrel on a magic box mag, you might be able to keep close to the usual efficiency of a mag with the lighter ball pressure. With flow dynamics, your pressure is proportional to the square root
of the flow volume. As you jack the pressure up, you're not getting proportionally higher flow. This means that you can increase the pressure behind the ball, and your leaks around the ball and bolt will not increase proportionally. The converse is also true. As you lower your pressure, the leaks will take a larger portion of your air mass.